Higher operating temperatures for gas turbine engines are continuously sought in order to increase efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. In this regard, materials containing silicon, particularly those with silicon carbide (SiC) as a matrix material or a reinforcing material, are currently being used for high temperature applications, such as for combustor and other hot section components of gas turbine engines, because of the excellent capacity of these silicon materials to operate at higher temperatures.
However, it has been found that silicon containing substrates can recede and lose mass as a result of a formation volatile Si species, particularly Si(OH)x and SiO when exposed to high temperature, aqueous environments. For example, silicon carbide when exposed to a lean fuel environment of approximately 1 ATM pressure of water vapor at 1200° C. will exhibit weight loss and recession at a rate of approximately 152.4 microns per 1000 hrs. It is believed that the process involves oxidation of the silicon carbide to form silica on the surface of the silicon carbide followed by reaction of the silica with steam to form volatile species of silicon such as Si(OH)x.
Methods such as described in U.S. Pat. No. 5,985,970 to Spitsberg et al., U.S. Pat. No. 6,410,148, U.S. Pat. No. 6,444,335 to Wang, et al, the disclosures of which are each all hereby incorporated by reference in their entirety, have dealt with the above problems concerning use of the silicon containing material substrates by providing a sufficient environmental barrier coating (EBC) for these substrates which inhibits the formation of volatile silicon species, Si(OH)x and SiO, thereby reduce recession and mass loss, and which provides thermal protection to and closely matches the thermal expansion of the silicon based substrate. In each of these systems, a conventional thermal barrier coating (TBC) comprising yttria stabilized zirconia (7% YSZ) is generally employed as a top layer to their respective EBC's in forming their TBC/EBC systems.
Nevertheless, as application temperatures increase beyond the thermal capability of a Si-containing material (limited by a melting temperature of about 2560° F. (about 1404° C.) for silicon), TBC/EBC systems utilizing conventional TBC's such as 7% YSZ may not adequately protect the underlying silicon containing material component. Namely, the thermal conductivities of TBC materials, such as YSZ, are known to increase over time when subjected to the operating environment of a gas turbine engine. In order for a TBC to remain effective throughout the planned life cycle of the component it protects, it is important that the TBC has and maintains a low thermal conductivity throughout the life of the component, including high temperature excursions. As possible solutions to these high temperature operating problems, TBCs for gas turbine engine components are often deposited to a greater thickness than would otherwise be desirable. Consequently, multiple layers are thus often added to some YSZ TBCs to correct deficiencies resulting in unwanted increased thickness of the coating system. Alternatively, internally cooled components, such as blades and nozzles, must be designed to have higher cooling flow. However, the above solutions may be undesirable for reasons relating to cost, weight, component life and engine efficiency.
In view of the above, it can be appreciated that further improvements in TBC technology are desirable, particularly as TBCs are employed to thermally insulate silicon containing material components intended for more demanding engine designs. A TBC having multiple beneficial effects, such as a low thermal conductivity, strong resistance to sintering, strong resistance to erosion, sufficiently long life and phase stability would allow for higher component surface temperatures and reduced coating thickness for the same surface temperature. Reduced TBC thickness, especially in applications like combustors often employing relatively thick TBCs, would result in a significant cost reduction and weight benefit. Additionally, thinner coatings on turbine blades and vanes would result in improved aerodynamic efficiencies and lower weight.